Receiver and reception method for estimating channel in an orthogonal frequency division multiple access system

ABSTRACT

A receiver and reception method for estimating a channel in an Orthogonal Frequency Division Multiple Access (OFDMA) system is provided. The receiver includes a delay estimator for estimating, from a signal received from a transmitter through multipaths, at least one of an average time delay of the multipaths and a time delay of one of the multipaths having a maximum power among the multipaths, a rotator for circular-rotating the received signal using the estimated delay, and a channel estimator for estimating a channel impulse response of the circular-rotated signal.

PRIORITY

This application claims the benefit under 35 U.S.C. §119(a) of a Koreanpatent application filed in the Korean Intellectual Property Office onJul. 31, 2007 and assigned Serial No. 2007-76988, the entire disclosureof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an Orthogonal Frequency DivisionMultiple Access (OFDMA) system. More particularly, the present inventionto a receiver and a reception method for estimating channels in an OFDMAsystem.

2. Description of the Related Art:

Orthogonal Frequency Division Multiplexing (OFDM) is a transmissionscheme that converts a serial input data stream into N parallel datastreams and carries the converted N parallel data streams on separateindividual subcarriers, thereby increasing a data rate. When the OFDMtransmission scheme is used for non-broadcast cellular mobilecommunication, wireless Local Area Network (LAN), wireless mobileInternet, etc., a Multiple Access scheme for multiple users is neededtogether with a single-carrier transmission scheme. Therefore, OFDMA isused as an OFDM-based Multiple Access scheme.

Orthogonal Frequency Division Multiple Access (OFDMA) is a scheme inwhich each user uses a number of subchannels and a number of OFDMsymbols. In the OFDMA scheme, the user allocates the number ofsubcarriers and the number of OFDM symbols differently according to thetransfer rate required by each user, thereby ensuring efficient resourcedistribution.

The terms ‘OFDM’ and ‘OFDMA’ will both be referred to herein as ‘OFDM’unless stated otherwise.

In the OFDM system, high-speed data transmission is required. For thehigh-speed data transmission, high-order modulation schemes (e.g.,16-ary Quadrature Amplitude Modulation (16-QAM) and 64-ary QAM (64-QAM))are needed. The high-order modulation scheme-based transmission methodexerts an influence on performance according to the channel state. Thatis, the high-order modulation scheme has a very high transfer rate in agood channel state but requires retransmission in a poor channel state,thus experiencing greater performance degradation when compared with thelow-order modulation schemes (e.g., Binary Phase Shift Keying (BPSK) andQuadrature PSK (QPSK)). Therefore, it is important to correctly estimatethe channel state and use a modulation scheme suitable thereto.

A method for estimating the channel state in the OFDM system will bedescribed. A transmitter transmits a base code or a pilot signalpreviously agreed upon with a receiver, and the receiver performschannel estimation using the base code or the pilot signal.

In the receiver of the OFDM system, a channel estimator greatly changesin performance according to frequency selectivity of the channel. Thefrequency selectivity is determined herein according to the time delay(or delay spread) characteristic of a multipath channel. That is, anincrease in the time delay of a channel causes an increase in thefrequency selectivity, and the increase in the frequency selectivityreduces channel estimation performance in the OFDM system using a pilotstructure.

FIG. 1 is a block diagram illustrating a structure of a receiver in aconventional OFDM system.

Since the signal received at a receiver from a transmitter is an analogsignal, an Analog-to-Digital Converter (ADC) 101 converts the receivedanalog signal into a digital signal. An Automatic Frequency Control(AFC) unit 103 cancels a frequency offset through frequency control onthe digital signal, and a Symbol Timing Recovery (STR) unit 105 sets anoptimal Fast Fourier Transform (FFT) window for the frequencyoffset-canceled received signal. An FFT unit 107 performs an FFT on areceived signal in the window to convert a time-domain received signalinto a frequency-domain received signal, and a channel estimator 109performs channel estimation by extracting a pilot signal from thefrequency-domain received signal. An equalizer 111 performs channelequalization on the frequency-domain received signal using a channelimpulse response estimated by the channel estimator 109. A Forward ErrorCorrection (FEC) unit 113 extracts information bits by performingchannel decoding on the channel-equalized input signal.

When the STR unit 105 in the conventional receiver of FIG. 1 sets an FFTwindow, Inter-Symbol Interference (ISI) may occur due to a precursor ofa channel.

FIG. 2 is a diagram illustrating an example in which ISI occurs duringan FFT window setting in the conventional STR unit of FIG. 1.

In the receiver, a received signal is expressed as a sum of severaltransmission signals 202 as shown in Equation (1), due to time delays201 of channels.

$\begin{matrix}\begin{matrix}{{y(n)} = {{h(n)}*{x(n)}}} \\{= {\left( {\sum\limits_{l = 0}^{L - 1}\; {h_{l}{\delta \left( {n - \tau_{l}} \right)}}} \right)*{x(n)}}} \\{= {\sum\limits_{l = 0}^{L - 1}\; {h_{l}{x\left( {n - \tau_{l}} \right)}}}}\end{matrix} & (1)\end{matrix}$

In Equation (1), h(n) denotes a channel impulse response, x(n) denotes atransmission signal corresponding to one OFDM symbol, y(n) denotes areceived signal corresponding to one OFDM symbol, and n denotes adiscrete time index. Further, in Equation (1), L denotes the number ofmultipaths, l denotes an index of a multipath, h_(l) denotes a channelimpulse response of each multipath, and τ_(l) denotes a time delay ofeach multipath.

To prevent ISI, the OFDM system inserts a Guard Interval (GI) 203 intoan OFDM symbol. In this case, the GI 203 generally has a longer lengththan the maximum time delay of the channel in the time domain.

With reference to FIG. 2 and under the assumption that the multipathchannel is composed of two multipaths, a description will be made of anexample in which ISI occurs due to a channel precursor.

Referring to FIG. 2, the STR unit 105 sets the part obtained byexcepting a GI from a received signal, as an FFT window 204 of an N^(th)OFDM symbol, on the basis of the path having the highest power among themultipaths. However, when a precursor exists in the channel, a part 205of an (N+1)^(th) OFDM symbol is included therein by the channelprecursor in the time-domain signal where the FFT window 204 of anN^(th) OFDM symbol is set, thereby generating ISI. In order to addressthis problem, the STR unit 105 time-advances, as shown by referencenumeral 206, an FFT window for an N^(th) OFDM symbol on the basis of themultipath that first occurred in the time domain among the multipaths,in consideration of a non-estimated additional channel precursor.

A received signal in an FFT window having N samples is defined asEquation (2).

$\begin{matrix}\begin{matrix}{{X(k)} = {{FFT}_{N}\left\{ {x(n)} \right\}}} \\{{= {\sum\limits_{n = 0}^{N - 1}\; {{x(n)} \cdot {\exp\left( {{- j}\frac{2\; \pi \; {kn}}{N}} \right)}}}},{k = {0,1}},\ldots \mspace{14mu},{N - 1}}\end{matrix} & (2)\end{matrix}$

If the FFT window is time-advanced by m samples to prevent ISI, x(n)undergoes circular rotation by m samples due to GI. An OFDM symbol thatunderwent m-sample circular rotation is defined as Equation (3) afterundergoing an FFT.

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{X}}_{m}(k)} = {{FFT}_{N}\left\{ {x_{N}\left( {n - m} \right)} \right\}}} \\{{= {{X(k)} \cdot {\exp\left( {j\frac{2\; \pi \; k\; m}{N}} \right)}}},{k = {0,1}},\ldots \mspace{14mu},{N - 1}}\end{matrix} & (3)\end{matrix}$

In Equation (3), x_(N)(n) means x(n), to which circular rotation using Nas a modulus is applied. That is, x_(N)(n) is expressed as Equation (4).

x _(N)(n)=x(n mod N), n=0,1, . . . ,N−1   (4)

When the FFT window is shifted by m samples along the time domain asshown in Equation (3), additional phase rotation occurs in thepre-shifting frequency response as shown in FIGS. 3 and 4.

FIGS. 3 and 4 are diagrams illustrating a channel impulse response and afrequency response for m samples in a conventional OFDM system.

Referring to FIG. 3, a channel impulse response and a frequency responseare illustrated for m=0, i.e. when an FFT window is set on the basis ofthe first multipath. Referring to FIG. 4 shows a channel impulseresponse and a frequency response are illustrated for m=4, i.e. when anFFT window is set 4 samples in advance of that for the first multipath.In FIGS. 3 and 4, making a comparison between a frequency response form=0 and a frequency response for m=4, the frequency response for m=4 ishigher in frequency selectivity since it is higher in a change rate thanthe frequency response for m=0.

In the conventional OFDM system, the receiver performs channelestimation with a method using pilot signal-based linear interpolationin order to reduce complexity. As a result, the frequency selectivity ofchannels has a direct influence on the channel estimation performance.That is, as in the examples of FIGS. 3 and 4, it is advantageous to setan FFT window after sufficiently shifting it forward while taking thechannel precursor into account, in order to prevent ISI. However, inthis case, channel estimation performance may be reduced in thesituation where no ISI occurs.

Therefore, there is a demand for a receiver and a reception method forsolving the channel estimation performance reduction problem caused bythe FFT window setting at the receiver of the conventional OFDM system,and for improving channel estimation performance of a channel having alarge time delay.

SUMMARY OF THE INVENTION

An aspect of the present invention is to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a receiver and reception method for improvingchannel estimation performance using time delay characteristics ofchannels.

According to one aspect of the present invention, a receiver forestimating a channel in an Orthogonal Frequency Division Multiple Access(OFDMA) system is provided. The receiver includes a delay estimator forestimating, from a signal received from a transmitter throughmultipaths, at least one of an average time delay of the multipaths anda time delay of one of the multipaths having a maximum power among themultipaths, a rotator for circular-rotating the received signal usingthe estimated delay, and a channel estimator for estimating a channelimpulse response of the circular-rotated signal.

According to another aspect of the present invention, a reception methodfor estimating a channel in an Orthogonal Frequency Division MultipleAccess (OFDMA) system is provided. The reception method includesestimating, from a signal received from a transmitter throughmultipaths, at least one of an average time delay of the multipaths anda time delay of one of the multipaths having a maximum power among themultipaths, circular-rotating the received signal using the estimateddelay, and estimating a channel impulse response of the circular-rotatedsignal.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram illustrating a structure of a receiver in aconventional OFDM system;

FIG. 2 is a diagram illustrating an example in which ISI occurs duringan FFT window setting in the conventional STR unit of FIG. 1;

FIGS. 3 and 4 are diagrams illustrating a channel impulse response and afrequency response for m samples in a conventional OFDM system;

FIG. 5 is a block diagram illustrating a structure of a receiveraccording to a first exemplary embodiment of the present invention;

FIG. 6 is a flowchart illustrating a reception method according to thefirst exemplary embodiment of the present invention;

FIG. 7 is a block diagram illustrating a structure of a receiveraccording to a second exemplary embodiment of the present invention;

FIG. 8 is a diagram illustrating an exemplary operation of the samplerotator of FIG. 7;

FIG. 9 is a flowchart illustrating a reception method according to thesecond exemplary embodiment of the present invention;

FIG. 10 is a diagram illustrating an impulse response and a frequencyresponse of channels, to which reverse sample rotation is applied by thequantized average time delay of channels, according to the secondexemplary embodiment of the present invention;

FIG. 11 is a block diagram illustrating a structure of a receiveraccording to a third exemplary embodiment of the present invention;

FIG. 12 is a diagram illustrating a reception method according to thethird exemplary embodiment of the present invention;

FIG. 13 is a diagram illustrating an impulse response and frequencyresponse of channels, to which reverse sample rotation is applied on thebasis of a delay value of the multipath having the maximum power,according to the third exemplary embodiment of the present invention;and

FIG. 14 is a diagram illustrating a comparison between the conventionalchannel estimation performance and the channel estimation performancebased on the first to third exemplary embodiments of the presentinvention.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. Also, descriptions of well-known functions and constructionsare omitted for clarity and conciseness. Terms used herein are definedbased on functions in the exemplary embodiments of the present inventionand may vary according to users, operators' intention or usualpractices. Therefore, the definition of the terms should be made basedon the contents throughout the specification.

In a receiver of an OFDM system, a conventional channel estimator usinga scheme such as pilot-based linear interpolation has a characteristicthat its performance reduces as frequency selectivity of channelsincreases. In addition, the frequency selectivity of channels isproportional to a time delay of channels. Therefore, a channel with alarge time delay, having a corresponding high frequency selectivity, iscomparatively inferior in channel estimation performance to the channelwith a small time delay. An average time delay of channels is defined asEquation (5).

$\begin{matrix}{\overset{\_}{\tau} = \frac{\sum\limits_{l = 0}^{L - 1}\; {{h_{l}}^{2}\tau_{l}}}{\sum\limits_{l = 0}^{L - 1}\; {h_{l}}^{2}}} & (5)\end{matrix}$

That is, an average time delay is calculated by dividing each multipathtime delay of channels by an average for channel power. As isconventionally known, each multipath time delay is calculated herein bya searcher for multipath reception.

The reason why the frequency selectivity of FIG. 4 is higher than thefrequency selectivity of FIG. 3 is because power distribution ofchannels is excessively distributed toward one side in the FFT window.Therefore, it is possible to minimize the frequency selectivity ofchannels by reversely performing circular rotation on the receivedsignal by an average time delay of channels according to an exemplaryembodiment of the present invention.

Methods for reversely performing circular rotation on the receivedsignal by an average time delay of channels according to an exemplaryembodiment of the present invention can be classified into threemethods. In the first method (first exemplary embodiment), phaserotation is performed on the received signal after an FFT in thefrequency domain. In the second method (second exemplary embodiment),sample rotation is performed on the received signal before an FFT in thetime domain. In the third method (third exemplary embodiment), reversecircular rotation is performed on the received signal before an FFT onthe basis of a delay value of the path having the maximum multipathpower, since a delay value of the path having the maximum multipathpower can be estimated with a method such as correlation.

First Exemplary Embodiment: Method for Phase-Rotating a Received SignalAfter an FFT

The first exemplary embodiment of the present invention phase-rotatesthe received signal after an FFT so that channel power of the receivedsignal can be symmetrically distributed on the basis of a time index 0of an FFT window, thereby minimizing a frequency selectivity ofchannels.

FIG. 5 is a block diagram illustrating a structure of a receiveraccording to the first exemplary embodiment of the present invention.

Referring to FIG. 5, since an ADC unit 501, an AFC unit 503, an STR unit505, a channel estimator 513, an equalizer 515, and an FEC unit 517 aresimilar in operation to the conventional ADC unit 101, AFC unit 103, STRunit 105, channel estimator 109, equalizer 111, and FEC unit 113 of FIG.1, a detailed description thereof will be omitted.

Since the first exemplary embodiment of the present inventionphase-rotates the received signal after an FFT, an FFT unit 507 performsan FFT on the received signal in one FFT window of Equation (1), asshown in Equation (6).

$\begin{matrix}\begin{matrix}{{Y(k)} = {{FFT}_{N}\left\{ {y(n)} \right\}}} \\{= {{FFT}_{N}\left\{ {{h(n)}*{x(n)}} \right\}}} \\{{= {{H(k)}{X(k)}}},{k = {0,1}},\ldots \mspace{14mu},{N - 1}}\end{matrix} & (6)\end{matrix}$

A time delay estimator 509 estimates an average time delay for thefrequency-domain received signal output from the FFT unit 507 on thebasis of the set FFT window using τ_(l) and h_(l) provided from the STRunit 505 as shown in Equation (5).

A phase rotator 511 performs reverse phase rotation on thefrequency-domain received signal output from the FFT unit 507 by theestimated average time delay, using Equation (7).

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{Y}}_{- \overset{\_}{\tau}}(k)} = {{Y(k)} \cdot {\exp\left( {j\frac{2\; \pi \; k\; \overset{\_}{\tau}}{N}} \right)}}} \\{= {{H(k)}{{X(k)} \cdot {\exp\left( {j\frac{2\; \pi \; k\; \overset{\_}{\tau}}{N}} \right)}}}} \\{{= {{{\overset{\sim}{H}}_{- \overset{\_}{\tau}}(k)}{X(k)}}},{k = {0,1}},\ldots \mspace{14mu},{N - 1}}\end{matrix} & (7)\end{matrix}$

The phase-rotated received signal is provided to the channel estimator513 and the equalizer 515 to be used for channel estimation andequalization, and the channel estimator 513 performs channel estimationby extracting a pilot signal from the phase-rotated received signal. Theequalizer 515 performs channel equalization on the phase-rotatedreceived signal using the channel impulse response estimated by thechannel estimator 513, and then provides the channel-equalized receivedsignal to the FEC unit 517.

FIG. 6 is a flowchart illustrating a reception method according to thefirst exemplary embodiment of the present invention.

In step 601, the ADC unit 501, AFC unit 503 and STR unit 505 convert areceived signal into a digital signal, perform frequency correction,perform timing correction, and set an FFT window, to output a receivedsignal in the FFT window corresponding to one OFDM symbol. In step 603,the FFT unit 507 applies an FFT to the received signal using Equation(6). In step 605, the time delay estimator 509 estimates an average timedelay for the received signal before application of an FFT using τ_(l)and h_(l) provided from the STR unit 505 as shown in Equation (5). Instep 607, the phase rotator 511 performs reverse phase rotation on theFFT-applied frequency-domain received signal using the estimated averagetime delay, according to Equation (7). In step 609, the channelestimator 513 and equalizer 515 perform channel estimation by extractinga pilot signal from the reversely phase-rotated received signal, andperform channel equalization on the reversely phase-rotated receivedsignal using the estimated channel impulse response. In step 611, theFEC unit 517 performs channel decoding on the channel-equalized inputsignal.

Second Exemplary Embodiment: Method for Sample-Rotating a ReceivedSignal Before an FFT

The second exemplary embodiment of the present invention sample-rotatesa received signal before an FFT so that channel power of the receivedsignal can be symmetrically distributed on the basis of a time index 0of an FFT window, thereby minimizing a frequency selectivity ofchannels.

FIG. 7 is a block diagram illustrating a structure of a receiveraccording to the second exemplary embodiment of the present invention.

Referring to FIG. 7, since an ADC unit 701, an AFC unit 703, an STR unit705, a channel estimator 715, an equalizer 717, and an FEC unit 719 aresimilar in operation to the ADC unit 101, the AFC unit 103, the STR unit105, the channel estimator 109, the equalizer 111, and the FEC unit 113of FIG. 1, a detailed description thereof will be omitted.

A time delay estimator 707 estimates an average time delay for thereceived signal in one FFT window based on τ_(l) and h_(l) provided fromthe STR unit 705 using Equation (5).

According to the second exemplary embodiment of the present invention, areverse circular rotation value should be an integer in order to applysample rotation in the time domain. However, the average time delaycalculated in Equation (5) may not be an integer and may instead be areal number. Accordingly, a quantizer 709 quantizes the average timedelay τ as an integer using Equation (8) in order to apply thetime-domain sample rotation.

τ _(q)=round( τ)   (8)

A sample rotator 711 performs time-domain sample rotation on thereceived signal before it is input to an FFT unit 713, by the quantizedaverage time delay as shown by way of an example in FIG. 8.

FIG. 8 is a diagram illustrating an exemplary operation of the samplerotator 711 in FIG. 7. In FIG. 8, when an average time delay is 2samples, the sample rotator 711 performs 2-sample rotation on thereceived signal on the basis of an FFT window 801 starting from a timeindex 0 in step 803. Since the number of samples included in one FFTwindow is 1024, the received signal after sample rotation is composed of1024 samples beginning from the 3^(rd) sample (sample #2) before samplerotation up to the second sample (sample #1).

The FFT unit 713 applies an FFT to the sample-rotated received signal.That is, the sample rotator 711 and the FFT unit 713 sample-rotate thereceived signal and then apply an FFT thereto using Equation (9).

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{Y}}_{- {\overset{\_}{\tau}}_{q}}(k)} = {{FFT}_{N}\left\{ {y_{N}\left( {n + {\overset{\_}{\tau}}_{q}} \right)} \right\}}} \\{{= {{{\overset{\sim}{H}}_{- {\overset{\_}{\tau}}_{q}}(k)}{X(k)}}},{k = {0,1}},\ldots \mspace{14mu},{N - 1}}\end{matrix} & (9)\end{matrix}$

The frequency-domain received signal output from the FFT unit 713 isprovided to the channel estimator 715 and the equalizer 717 so as to beused for channel estimation and equalization, and the channel estimator715 performs channel estimation by extracting a pilot signal from thefrequency-domain received signal. The equalizer 717 performs channelequalization on the phase-rotated received signal using the channelimpulse response estimated by the channel estimator 715, and thenprovides the result to the FEC unit 719.

FIG. 9 is a flowchart illustrating a reception method according to thesecond exemplary embodiment of the present invention.

In step 901, the ADC unit 701, AFC unit 703 and STR unit 705 convert areceived signal into a digital signal, perform frequency and timingcorrection, and then set an FFT window, to output a received signal inthe FFT window corresponding to one OFDM symbol. In step 903, the timedelay estimator 707 estimates an average time delay for the receivedsignal before application of an FFT, using Equation (5). In step 905,the quantizer 709 quantizes the average time delay as an integer usingEquation (8) in order to apply time-domain sample rotation. In step 907,the sample rotator 711 performs reverse sample rotation on the receivedsignal using the quantized average time delay. In step 909, the FFT unit713 applies an FFT to the sample-rotated received signal. In step 911,the channel estimator 513 and equalizer 515 perform channel estimationby extracting a pilot signal from the frequency-domain received signaloutput from the FFT unit 713, and perform channel equalization on thefrequency-domain received signal using the estimated channel impulseresponse. In step 913, the FEC unit 517 performs channel decoding on thechannel-equalized input signal.

The impulse response and frequency response of channels to which reversesample rotation is applied by the quantized average time delay ofchannels according to the second exemplary embodiment of the presentinvention are as shown in FIG. 10. It can be seen from FIG. 10 that thefrequency selectivity according to the second exemplary embodiment ofthe present invention is lower than the frequency selectivity of FIGS. 3and 4. Therefore, the second exemplary embodiment of the presentinvention can improve channel estimation performance at the receiver.

Third Exemplary Embodiment: Method for Reversely Sample-Rotating aReceived Signal Before an FFT on a Basis of a Delay Value of aMaximum-Power Path

The power and delay value of each multipath should be estimated for acalculation of an average time delay in Equation (5), and in some cases,estimation of the power and delay value of each multipath is verydifficult. However, since a delay value of the multipath having themaximum power can be easily estimated with a method such as correlation,reverse sample rotation is applied on the basis of the delay value ofthe multipath having the maximum power according to the third exemplaryembodiment of the present invention.

FIG. 11 is a block diagram illustrating a structure of a receiveraccording to the third exemplary embodiment of the present invention.

Referring to FIG. 11, since an ADC unit 1101, an AFC unit 1103, an STRunit 1105, a channel estimator 1113, an equalizer 1115, and an FEC unit1117 are similar in operation to the conventional ADC unit 101, AFC unit103, STR unit 105, channel estimator 109, equalizer 111, and FEC unit113 of FIG. 1, a detailed description thereof will be omitted.

A maximum estimator 1107 estimates a delay value of a path of themultipaths having the maximum power for the received signal in one FFTwindow on the basis of the set FFT window using τ_(l) and h_(l) providedfrom the STR unit 1105 as shown in Equation (10).

$\begin{matrix}{l_{\max} = {\max\limits_{l}{h_{l}}^{2}}} & (10)\end{matrix}$

A sample rotator 1109 performs sample rotation on the received signalusing the estimated delay value. An FFT unit 1111 applies an FFT to thesample-rotated received signal. That is, the sample rotator 1109 and theFFT unit 1111 sample-rotate the received signal, and then apply an FFTthereto using Equation (11).

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{Y}}_{- \tau_{l\; \max}}(k)} = {{FFT}_{N}\left\{ {y_{N}\left( {n + \tau_{l\; \max}} \right)} \right\}}} \\{{= {{{\overset{\sim}{H}}_{- \tau_{l\; \max}}(k)}{X(k)}}},{k = {0,1}},\ldots \mspace{14mu},{N - 1}}\end{matrix} & (11)\end{matrix}$

The frequency-domain received signal output from the FFT unit 1111 isprovided to the channel estimator 1113 and equalizer 1115 to be used forchannel estimation and equalization, and the channel estimator 1113performs channel estimation by extracting a pilot signal from thefrequency-domain received signal. The equalizer 1115 performs channelequalization on the phase-rotated received signal using the channelimpulse response estimated by the channel estimator 1113, and thenprovides the result to the FEC unit 1117.

FIG. 12 is a diagram illustrating a reception method according to thethird exemplary embodiment of the present invention.

In step 1201, the ADC unit 1101, AFC unit 1103 and STR unit 1105 converta received signal into a digital signal, perform frequency and timingcorrection, and then set an FFT window, to output a received signal inthe FFT window corresponding to one OFDM symbol. In step 1203, themaximum estimator 1107 estimates a delay value of the path of themultipaths having the maximum power from the received signal on thebasis of the set FFT window using τ_(l) and h_(l) provided from the STRunit 1105 as shown in Equation (10). In step 1205, the sample rotator1109 performs sample rotation on the received signal using the estimateddelay value. In step 1207, the FFT unit 1111 applies an FFT to thesample-rotated received signal. In step 1209, the channel estimator 1113and equalizer 1115 perform channel estimation by extracting a pilotsignal from the FFT-applied frequency-domain received signal, andperform channel equalization on the frequency-domain received signalusing the estimated channel impulse response. In step 1211, the FEC unit1117 performs channel decoding on the channel-equalized input signal.

The impulse response and frequency response of channels to which reversesample rotation is applied on the basis of the delay value of themultipath having the maximum power according to the third exemplaryembodiment of the present invention are as shown in FIG. 13. It can beseen from FIG. 13 that the frequency selectivity according to the thirdexemplary embodiment of the present invention is lower than theconventional frequency selectivity since the frequency responseaccording to an exemplary embodiment of the present invention is lowerin a change rate than the conventional frequency response. Therefore,the third exemplary embodiment of the present invention can improvechannel estimation performance at the receiver.

A comparison between the channel estimation performance at the receiveraccording to the first through third exemplary embodiments and thechannel estimation performance at the conventional receiver isillustrated in FIG. 14.

FIG. 14 is a diagram illustrating a comparison between the conventionalchannel estimation performance and the channel estimation performancebased on the first through third exemplary embodiments of the presentinvention. In FIG. 14, a comparison between the channel estimationperformance at the conventional receiver and the channel estimationperformance at the receiver based on the first through third exemplaryembodiments of the present invention is made using Mean Squared Error(MSE) calculated as shown in Equation (12).

$\begin{matrix}{{{MSE}\mspace{14mu} ({dB})} = {10*\log \; 10\left( {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}\; {{{\overset{\sim}{H}(k)} - {H(k)}}}^{2}}} \right)}} & (12)\end{matrix}$

where {tilde over (H)}(k) denotes channel estimation results based onthe conventional art and the first through third exemplary embodimentsof the present invention, and H(k) denotes an ideal frequency responseof channels.

In the comparison illustrated in FIG. 14, it is assumed that pilotsamples are inserted into 128 carriers from among 1024 carriers in anOFDM symbol at regular intervals, a simple linear interpolation methodusing 128 pilot samples is used for channel estimation, and the usedchannel environment is an ITU-R Pedestrian-B channel.

As can be understood from the comparison illustrated in FIG. 14, thefirst and second exemplary embodiments (Method 1 and Method 2) of thepresent invention exhibit substantially the same channel estimationperformance which is also the best performance. The third exemplaryembodiment (Method 3) of the present invention is not superior to thefirst and second exemplary embodiments of the present invention, but issuperior to the conventional channel estimation scheme (Conventional).In terms of complexity, the first exemplary embodiment of the presentinvention requires complex multiplication after an FFT, but the secondand third exemplary embodiments are comparatively simple since they onlyneed to use the sample rotators 711 and 1109 before an FFT.

Therefore, in consideration of performance and complexity, it is idealto implement the receiver according to the second exemplary embodimentof the present invention if implementation of the time delay estimationis easy, and it is ideal to implement the receiver according to thethird exemplary embodiment of the present invention if implementation ofthe time delay estimation is difficult.

As is apparent from the foregoing description, exemplary embodiments ofthe present invention can address the channel estimation performancereduction problem caused by an FFT window setting at a receiver of anOFDM system, and improve the channel estimation performance even in achannel having a large time delay.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

1. A receiver for estimating a channel in an Orthogonal FrequencyDivision Multiple Access (OFDMA) system, the receiver comprising: adelay estimator for estimating, from a signal received from atransmitter through multipaths, at least one of an average time delay ofthe multipaths and a time delay of one of the multipaths having amaximum power among the multipaths; a rotator for circular-rotating thereceived signal using the estimated delay; and a channel estimator forestimating a channel impulse response of the circular-rotated signal. 2.The receiver of claim 1, wherein the delay estimator estimates theaverage time delay of a channel using the following equation;$\overset{\_}{\tau} = \frac{\sum\limits_{l = 0}^{L - 1}\; {{h_{l}}^{2}\tau_{l}}}{\sum\limits_{l = 0}^{L - 1}\; {h_{l}}^{2}}$where h_(l) denotes a channel impulse response of each multipath, τ_(l)denotes a time delay of each multipath, L denotes the number ofmultipaths, and l denotes an index of a multipath.
 3. The receiver ofclaim 1, wherein the rotator reversely circular-rotates a phase of thereceived signal by the estimated delay when the received signal is afrequency-domain signal after a Fast Fourier Transform (FFT).
 4. Thereceiver of claim 1, wherein the rotator quantizes the estimated delayand reversely circular-rotates samples of the received signal by thequantized delay, when the received signal is a time-domain signal beforean FFT.
 5. The receiver of claim 4, wherein the estimated delay isquantized as an integer.
 6. The receiver of claim 1, wherein the channelestimation by the channel estimator comprises extracting a pilot signal.7. The receiver of claim 1, wherein the circular-rotating of thereceived signal comprises reversely circular-rotating the receivedsignal.
 8. A reception method for estimating a channel in an OrthogonalFrequency Division Multiple Access (OFDMA) system, the methodcomprising: estimating, from a signal received from a transmitterthrough multipaths, at least one of an average time delay of themultipaths and a time delay of one of the multipaths having a maximumpower among the multipaths; circular-rotating the received signal usingthe estimated delay; and estimating a channel impulse response of thecircular-rotated signal.
 9. The reception method of claim 8, wherein theaverage time delay of a channel is estimated using the followingequation;$\overset{\_}{\tau} = \frac{\sum\limits_{l = 0}^{L - 1}\; {{h_{l}}^{2}\tau_{l}}}{\sum\limits_{l = 0}^{L - 1}\; {h_{l}}^{2}}$where h_(l) denotes a channel impulse response of each multipath, τ_(l)denotes a time delay of each multipath, L denotes the number ofmultipaths, and l denotes an index of a multipath.
 10. The receptionmethod of claim 8, wherein circular-rotating the received signalcomprises: reversely circular-rotating a phase of the received signal bythe estimated delay when the received signal is a frequency-domainsignal after a Fast Fourier Transform (FFT).
 11. The reception method ofclaim 8, wherein circular-rotating the received signal comprises:quantizing the estimated delay and reversely circular-rotating samplesof the received signal by the quantized delay, when the received signalis a time-domain signal before an FFT.
 12. The reception method of claim11, wherein the quantizing of the estimated delay comprises quantizingthe estimated as an integer.
 13. The reception method of claim 8,wherein the channel estimation comprises extracting a pilot signal. 14.The reception method of claim 8, wherein the circular-rotating of thereceived signal comprises reversely circular-rotating the receivedsignal.